Incorporating the appropriate window shades over certain windows can make a substantial difference in the aesthetics, comfort and energy savings in a room. In that regard, window shades are often utilized for a variety of purposes such as, for example, maximizing a view, maximizing daylight, blacking out a room, minimizing brightness, adjusting to climatic variables and sky conditions, minimizing heat at certain times of the year, maximizing heat during other times of the year, protecting work surfaces, minimizing glare and protecting people from direct sun.
When choosing the appropriate window shade system, lighting designers and interior architects typically consider various factors such as, for example, glazing (glass) properties, room properties and environmental conditions. The glass properties may include total solar and visible properties (transmission, reflection, absorption), single or multiple pane glass, chemicals or materials between the panes of glass, angled glass, tint, sun screens, UV transmission, bars over the windows, frosted glass and the like. The room properties may include the interior lighting and the reflectance from the wall, floor and ceiling. The environmental conditions may include typical solar or climate conditions (e.g., often cloudy in Seattle, often clear skies in Phoenix, etc.), obstructions (mountains, trees, other buildings, etc.), luminance (the amount of light leaving a point on a surface in a given direction (e.g., that comes to the eye from a surface)) and illuminance (the amount of visible light on a surface from all directions above that surface or the density of luminous flux incident on a surface). Luminance is measured in Footlamberts, Candala/Square Meter, Nits, or Lambert. Illuminance is measured in Lux, Footcandle or Lumen/Square Meter. The impact of a window shade system on a particular room may also be calculated under various conditions. For example, measurements may be obtained related to the luminance from the shade, walls and ceiling, the amount of light through the shade and glass, etc.
In recent years, corporate and institutional building design includes higher and higher visual light transmission glass for allowing more natural daylight into the building space, enhancing the view to the outside and using the daylight to reduce artificial lighting and A/C energy usage. Such increasing use of higher visual light transmission glass creates both problems and opportunities.
Since around the year 2000, designers have changed their selection of building glazings to low E clear glass, Starfire (no iron) glass, or a similar tinted low E glazing. Such glazings have the highest ratio of visible light transmission (VLt) to Solar Heat Gain Factor (SHGF). Over the years, the VLt of double glazing has changed from a low of 0.20 (low E solar ban), to an uncoated bronze or gray heat absorbing glass of 0.40 VLt, to low iron glass (green, blue aqua) with a VLt of 0.6 and now to a low E clear on Starfire glass with a VLt of 0.70-0.75. The SHGF percentage of heat inside the glass with a low E coating has remained at +/−0.40-0.55. In other words, the ratio of heat gain to VLt through the glass was previously close to a 1:1 ratio, but the ratio is now +/−1:1.75, which is a dramatic increase of VLt over heat gain.
Based in part on the lower heat gain, the HVAC systems have been down sized. However, the HVAC systems have not been sufficiently adjusted for the substantial gain in VLt which also has a strong direct solar radiant component.
Moreover, in an effort to reduce glare and limit the impact of transient adaptation of the eye as the eye goes from one area (the task) to another area (adjacent surroundings or surfaces), lighting designers have determined an appropriate ratio for the perceived and measured brightness inside a person's field of view. Adjacent surfaces are within a 30 degree visual cone and non-adjacent surfaces are within a 30-60 degree cone. (a person's field of view is generally considered to be a 60 degree visual cone). The recommended ratio between task and adjacent surroundings is 3:1, the recommended ratio between task and remote surroundings is 10:1, and the recommended ratio should be 40:1 for everything outside the 60 degree cone. These ratios are applicable for areas on the order of one steradian, so higher ratios are recommended in small areas to add visual interest.
To provide an example of the ratios, if the work surface has 50 foot candles (FC), then the visual zone should not have any glare or brightness that exceeds 500 FC within remote surroundings, namely within the 1:10 ratio. In the case of a computer screen, if the lumen output of the screen is 200 Candela M/2, then the maximum amount of brightness in the person's field of view should not be more than 2000 Candela M/2 within remote surroundings.
As the VLt of the glass increases and as lighting designers attempt to control the brightness inside a person's field of view, the shade cloth color on the outside of the building also has an effect on the building design, along with the uniformity and alignment of the window covering. As such, the impact of the window covering is now becoming an integral element in building design.
To calculate the heat flow through the glass by convection and direct radiation, an ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.) formula using the solar optical properties of the fabric is used to calculate SHGF of the glass, along with the glass and shade combination. Currently, the available performance data for a screen fabric are Total Solar Reflectance (TSr); Total Solar Transmission (TSt); and Total Solar Absorbtion (TSa), wherein the total of TSr+TSt+TSa=100%. Other performance data includes Visible Light Transmission; UV Transmission and Openness Factor (Fabric Density); however, Visible Light Transmission; UV Transmission and Openness Factor are not included in the ASHRAE formula as they are not components of the total solar measurements used above to develop the SHGF. The SHGF provides the necessary information for calculating heat flow into the building to enable mechanical engineers to more effectively size the HVAC systems.
However, the SHGF does not address the comfort factors of direct solar radiation, or visual brightness near the window wall (i.e., 15-20 feet from the window wall). The engineering standards of the shading coefficient, solar heat gain factor, or solar factor do not include a valuation of comfort at the window wall for the occupant. As such, the tests were conducted to determine the factors that effect personal comfort near a window wall with sun screens. The tests matched different types and kinds of glass with a variety of woven sun screen fabrics, then measured the total heat gain, solar radiation, heat gain and visible light transmission. The tests resulted in a method for determining a screen cloth's “personal comfort value” under reasonable interior environmental conditions with glazing of a specific VLt and/or SHGF.
While various factors, tests and calculations exist for determining the optimum window system, the SHGF and the personal comfort values still do not include the relative brightness (illuminance) of the fabric when it is solar lit, its effect on the interior environment and its impact on the viewers. A strong need exists to compare the surface brightness of different fabrics with a uniform light source to determine the relative brightness of one screen fabric to another screen fabric. A strong need also exists to determine the illuminance or brightness value of a screen on the project such that the factor can help determine the optimum window shade fabric for a particular room, building or other enclosure.